CBSE Class 12 Biology Organisms and Populations Assignment Set C

Very Short Answer Type Questions

Question. Name the type of interaction seen between fig and wasps.
Answer: Mutualism.

Question. Mention the term used to describe a population interaction between an orchid growing on a forest tree.
Answer: Commensalism.

Question. State the type of interaction that exists between ticks and dogs.
Answer: Ectoparasitism.

Question. In a pond there were 200 frogs. 40 more frogs were born in a year. Calculate the birth rate of the population.
Answer: Birth rate = 40/200 = 0.2 frogs/yr or 20%

Question. Name the type of interaction that exists between barnacles and whale.
Answer: Commensalism

Question. Name a ‘photoperiod’ dependent process, one each in plants and in animals.
Answer: In plants, flowering and in animals, migration/foraging are photoperiod dependent processes.

Question. Between amphibians and birds, which will be able to cope with global warming? Give reasons. 
Answer: Birds being eurythermals can tolerate a wide range of temperature and thus will be able to cope with global warming more efficiently.

Question. An orchid plant is growing on the branch of mango tree. How do you describe this interaction between the orchid and the mango tree?
Answer: The interaction between an orchid and the mango tree is commensalism, because orchid is benefited by getting shelter from mango tree whereas the mango tree is neither harmed nor benefited.

Question. Why are some organisms called as eurythermals and some others as stenohaline?
Answer: Eurythermals are organisms that can tolerate and thrive in a wide range of temperature, whereasste nohalines can tolerate a narrow range of salinities.

Question. Categorise the following plants into hydrophytes, xerophytes, halophytes and mesophytes. Write the type of plant against the following examples.
(a) Salvinia (b) Opuntia (c) Rhizophora (d) Mangifera 
Answer: (a) Hydrophyte (b) Xerophyte (c) Halophyte (d) Mesophyte

Question. In a pond, we see plants which are free-floating, rooted-submerged, rooted emergent, rooted with floating leaves. Write the type of plants against each of them.
(a) Hydrilla, (b) Typha, (c) Nymphaea, (d) Lemna, (e) Vallisnaria 
Answer: a) Submerged (b) Rooted emergent (c) Rooted with floating leave (d) free-floating (e) Rooted Submerged

Question. The density of a population in a habitat per unit area is measured in different units. Write the unit of measurement against the following:
(a) Bacteria, (b) Grass, (c) Banyan, (d) Deer, (e) Fish 
Answer: (a) Numbers/volume (b) Coverage/area (c) Biomass/area (d) Numbers/area (e) Weight/area or Number/area

Question. How do kangaroo rats and desert plants adapt themselves to survive in their extreme habitat? Explain.
Answer: Kangaroo rats : Internal fat oxidation where water is a byproduct, excretes concentrated urine.
Desert Plants : Thick cuticle / sunken stomata / leaves reduced to spines / deep roots / Special photosynthetic pathway / CAM

Question. What is mutualism ? Mention any two examples where the organisms involved are commercially exploited in agriculture.
Answer: Interaction between two species in which both are benefitted 
(i) Rhizobium in the roots (nodules) of legumes 
(ii) Mycorrhiza / Glomus with the roots of higher plants.

Question. Heat loss or heat gain depends upon the surface area of the organism’s body. Explain with the help of a suitable example.
Answer: Small animal such as humming bird / shrew, have a larger surface area relative to their volume, they tend to lose heat when it is cold outside, hence spend much energy to generate body heat through metabolism.

Question. Differentiate between an ‘Expanding age pyramid’ and a ‘Stable age pyramid’.
Substantiate your answer with diagrams.
Answer: (Image 151)

Question. “Snow leopards are not found in Kerala forests and tuna fish are rarely found beyond tropical latitude in the ocean”. Study the above two cases and state the possible reasons for the same.
Answer: Change in temperature from their established habitats affects the kinetics of the enzymes and through it, the basal metabolism, activity and other physiological functions of the organism.

Question. Why are coral reefs not found from West Bengal to Andhra Pradesh but found in Tamil Nadu on the east coast of India?
Answer: High salinity, optimal temperature and less siltation are essential to colonise corals. If siltation and fresh water inflow are very high, the corals don’t colonise. In contrast when the siltation and fresh water in flow by the rivers are very less, the corals do colonise.

Question. In a sea shore, the benthic animals live in sandy, muddy and rocky substrata and accordingly developed the following adaptations. Find the suitable substratum against each adaptation.
(a) Burrowing ______________________
(b) Building cubes ______________________
(c) Holdfasts/peduncle ______________________ 
Answer: (a) Sandy, (b) Muddy, (c) Rocky.

Question. Which one of the two, stenothermals or eurythermals, shows wide range of distribution on earth and why?
Answer: Eurythermals show a wide range of distribution on earth, as they show tolerance for wide range of temperatures.

Question. In a pond there were 20 Hydrilla plants. Through reproduction 10 new Hydrilla plants were added in a year. Calculate the birth rate of the population. 
Answer: Birth rate = Number of individuals born /Total number of individuals = 10/20 = 0.5 Birth rate is 0.5 plants per year.

Question. Mention the effect of global warming on the geographical distribution of stenothermals like amphibians. 
Answer: Due to global warming, stenothermals would either migrate or die due to change in the temperature.

Question. Give an example for:
(a) An endothermic animal (b) Ectothermal animal
(c) An organism of benthic zone.
Answer: (a) Monkey (b) Snake (c) Angler fish.

Question. When and why do some animals go into hibernation?
Answer: When the animals are not able to tolerate the stressful conditions like low temperature, they hibernate to avoid the stress by escaping in time since they can not migrate.

Question. (i) Compare, giving reason, the J-shaped and S-shaped models of population growth, of a species.
(ii) Explain ”fitness of a species” as mentioned by Darwin.
Answer: (i) (Image 156)

Question. Select the statement which explains parasitism best.
(a) One organism is benefited.
(b) Both the organisms are benefited.
(c) One organism is benefited, other is not affected.
(d) One organism is benefited, other is affected.
Answer: D

Short Answer Type Questions

Question. How do mammals living in colder regions and seals living in polar regions able to reduce the loss of their body heat ?
Answer: Mammals from colder climates generally have shorter ears and limbs which minimise heat loss (Allen’s rule).
In the polar region, seals have a thick layer of fat (blubber) below their skin that acts as an insulator and reduce loss of body heat.

Question. Apart from being part of the food chain,predators play other important roles. Mention any two such roles supported by examples.
Answer: (i) Predators act as conduits for energy transfer across trophic levels.
(ii) They keep prey population under control.
(iii) They help in maintaining species diversity in a community by reducing intensity of competition among competing prey species.
(iv) An efficient predator may cause extinction of prey species.

Question. How is the normal human body temperature of 37°C maintained during (i) Summer, and
(ii) Winter? Explain.
Answer: Human beings maintain a constant temperature of 37°C.
In summers, the outside temperature is much higher than our body temperature. Therefore, we sweat profusely. This results in evaporative cooling, and our body temperature is reduced to normal.
In winters, the outside temperature is much lower than our body temperature. Therefore, we shiver, which is a kind of exercise, which produces heat and raises the body temperature.

Question. Plants that inhabit a rain-forest are not found in a wetland. Explain.
Answer: Plants that inhabit a rain forest are not adapted to survive in aquatic conditions / wetlands.
Plants are conformers / stenothermal / cannot maintain constant internal environment/ temperature / osmotic concentration of the body fluids affects kinetics of enzymes through basal
metabolism / activity and other physiological functions of the organisms

Question. Why do people suffer from altitude sickness after reaching the high altitude regions ? How does their body acclimatize after a couple of days ?
Answer: “Altitude sickness” is because of low atmospheric pressure at high altitude, the body does not get sufficient oxygen.
The body compensates low oxygen availability by increasing RBCs production, decreasing the binding capacity of haemoglobin, by increasing breathing rate.

Question. The graph given below shows the distribution of biomes : (Image 151)
(a) What do the ‘X’ and ‘Y’ axes represent?
(b) Identify the ‘grassland’ and ‘coniferous forest’ biomes, from the above figure.
(c) Why is ‘F’ located at the given position in the graph?
Answer: (a) ‘X’ axis–Mean annual precipitation (cm)
‘Y’ axis–Mean annual temperature (0°C)
(b) Grassland – B
Coniferous forest – E 
(c) The mean annual temperature ranges from – 12 to 20°C (error accepted ± 2) and mean annual precipitation ranges from 10 – 125 cm,these are the optimum conditions in tundra biome.

Question. Are hummingbirds and fish, regulators or conformers ? Give reasons in support of your answer.
Answer: Conformers. 
Heat loss or gain is a function of surface area. 
Since small animals have a larger surface area
(relative to their volume), they tend to lose body heat very fast when it is cold outside, they have to expend much energy, to generate body heat through metabolism.
(cannot maintain a constant body temperature).

Question. Explain by taking three different examples how do certain organisms pull through the adverse conditions when unable to migrate under stressful period.
Answer: Hibernation—winter sleep to escape cold weather e.g., bears.
Aestivation—summer sleep to avoid heat and desiccation e.g., snails / fish.
Diapause—suspended development / activity e.g., zooplanktons.
Spore formation—to tide over unfavourable conditions e.g., fungi / bacteria / lower plants.
Dormancy—By reducing metabolic activity e.g., seeds.

Question. (a) List any three ways of measuring population density of a habitat.
(b) Mention the essential information that can be obtained by studying the population density of an organism. 
Answer: (a) By physical counting, percent cover or total biomass, from relative density, counting pugmarks, counting faecal pellets. (Any three)
(b) Status of habitat, whether competition for survival exists or not, whether population is increasing or declining, natality, mortality, emigration, immigration.

Question. (a) The graph given below represents the organisms response to temperature as an environmental condition. (Image 156)
(i) Which one of the two lines represents conformers and why ?
(ii) What does the other line in the graph represent and why ?
(b) Mention the different adaptations the parasites have evolved with, to be able to successfully complete their life cycles in their hosts.
Answer: (a) (i) A is a conformer 
Cannot maintain homeostasis / constancy of internal environment by physiological means / their body temperature and osmotic concentration of body fluids changes with the ambient temperature. 
(ii) Regulators 
Maintain homeostasis by physiological means / capable of thermoregulation / maintain a constant body internal environment.
(b) Loss of unnecessary sense organs, presence of adhesive organs / suckers to cling on to the host, loss of digestive system, high reproductive capacity.

Question. Given below is a table depicting population interactions between species A and species B. (Image 154)
Name the types of interactions (a) and (b) in the above table.
Justify giving three reasons, how the type of interaction (b) is important in an ecological context. 
Answer: (a) – Amensalism 
(b) – Predation 
Justifications :
• Nature’s way of transferring energy fixed by plants to higher trophic levels/conduits for energy transfer.
• Keep prey population under control. 

Question. (a) Explain giving reasons why the tourists visiting Rohtang Pass or Mansarovar are advised to resume normal ‘active life’ only after a few days of reaching there.
(b) It is impossible to find small animals in the polar regions. Give reasons. 
Answer: (a) Initially the person suffers from altitude sickness/nausea, fatigue and heart palpitation because of low oxygen availability and low atmospheric pressure. Gradually the body increases RBC production, decreasing binding capacity of Hb and increases the breathing rate to get acclimatised.
(b) Small animals have larger surface area relative to their volume, so they lose heat much faster, & have to spend more energy to generate body heat.

Question. How do kangaroo rats and desert plants adapt themselves to survive in their extreme habitat? Explain.
Answer: Kangaroo rats are capable of meeting its water requirements through its internal fat oxidation in which water is a by product. It also has the ability to concentrate its urine so that minimal volume of water is used to remove excretory products. Desert plants have a thick cuticle on their leaf surface and have their stomata arranged in deep pits to minimise water loss. They also have leaves reduced to spines and deep roots to absorb more water. They have a special photosynthetic pathway (CAM).

Question. (a) Name the two growth models that represent population growth and draw the respective growth curves they represent.
(b) State the basis for the difference in the shape of these curves.
(c) Which one of the curves represent the human population growth at present? Do you think such a curve is sustainable? Give reason in support of your answer.
Answer: (a) Exponential growth curve and logistics growth curve For graph refer (b) The difference in the shape of the curve is due to the amount of resources available for the given population. When resources are unlimited, each species realises its innate potential to grow in number and result in a J-shaped curve in exponential growth while in logistics growth no population has unlimited resources leading to competition for resources and show S-shaped curve.
(c) Logistic growth represents human population growth at present. Such a curve is not sustainable because with growing population natural resources are getting depleted and its availability is not increasing enough.

Question. Predation is usually referred to as a detrimental association. State any three positive roles that a predator plays in an ecosystem. 
Answer: (i) They predators act as conduits for energy transfer across trophic levels.
(ii) They keep prey populations under control.
(iii) They help in maintaining species diversity in a community by reducing the intensity of competition among prey species.

Question. Explain co-evolution.
Answer: Co-evolution can be defined as reciprocal adaptations in two interacting organisms that brings about the evolutionary change in both of them. In terms of the relation of host and parasite,it can be explained as follows :
Parasite is an organism that is totally dependent on the host organism for its survival but in doing so, it also harms the host. The host evolved over a long period of time to protect itself from parasite, while the parasite evolved so that it can find another way to derive nutrition from the host and hence, the cycle continues.
The various parasitic adaptation enabling it to live in host are as follows :

Question. Explain relationship between biotic potential and environmental resistance.
Answer: Biotic potential is defined as the maximum inherent capacity of an organism to reproduce or increase the number of individuals. Whereas the environmental resistance is the biotic and abiotic factors of the environment, that do not allow the population of organisms to grow unlimited and keeps the population size in control.

Question. Name two basic types of competition found amongst organisms. Which one of them is more intense and why?
Answer: The two basic types of competitions are:
(i) Interspecific competition
(ii) Intraspecific competition
The intraspecific competition is more intense because the requirement of the individual of the species are similar.

Question. Mention four adaptive features that help cacti survive in xeric environment. 
Answer: Adaptation in desert plants:
(i) Desert plants have thick waxy coating on leaf called cuticle for minimum loss of water, through transpiration.
(ii) They have special photosynthetic pathway (CAM) that enables minimum loss of water during day time because stomata remain closed.
(iii) Some desert plants develop spines instead of leaf and photosynthetic function is carried out by the flattened stem.
(iv) Stomata are arranged in deep pits to minimise loss, through transpiration.

Question. List the attributes that populations, but not individuals possess.
Answer: The attributes that populations but not individuals possess are:
(i) Population density (ii) Population growth
(iii) Mortality or death rate (iv) Natality or birth rate
(v) Sex ratio (vi) Age distribution

Long Answer Type Questions

Question. (i) ”Organisms may be conformers or regulators.” Explain this statement and give one example of each.
(ii) Why are there more conformers than regulators in the animal world ?
                                            ​​​​​​​                      OR
Why are certain organisms called regulators or conformers ? Explain with the help of one example of each.
Answer: (i) Regulators maintain / regulate constant body temperature irrespective of external condition. 
Conformers changing / varying body temperature as per external condition.  
Regulators : Humans maintain a constant body temperature of 37°C. In summer when temperature is more outside we sweat, when evaporates causes cooling / in winter when
outside temperature is much lower than 37°C,we start shivering / a kind of exercise which produce heat and raises body temperature (any other suitable appropriate example).
Conformers : In aquatic animals, the osmotic concentration of body fluid changes with that of the ambient water osmotic concentration / Internal body temperature of reptiles,
amphibians, fishes change with that of external temperature. (Any other suitable appropriate example.)
(ii) Thermoregulation is energetically expensive for animals.

Question. Mention the special adaptations evolved in parasites and why ?
Answer: – Loss of unnecessary sense organs, since they do not interact with external environment (e.g.,lacks eyes as they are found in an environment that lacks light).
– Presence of adhesive organs / suckers / hooks, to cling to the host.
– Loss of digestive system , to absorb (digested) food from the host body. 
– High reproductive capacity , to increase the chances of the survival.
– If the host evolves special mechanism for resisting or rejecting the parasite -the parasite also evolves mechanism to counteract and neutralise them , in order to be successful with the same host species.
– Presence of more than one host , to facilitate parasitisation of its primary host.
– Loss of chlorophyll & leaves (Cuscuta) , to derive its nutrition from the host plant which it parasitise.
– Eggs resembles the host egg (crow) in size and colour , to reduce the chances of host bird detecting / ejecting the foreign eggs (koel).

Question. Mention how have plants developed mechanical and chemical defence against herbivores to protect themselves with the help of one example of each.
Answer: Several plants have evolved various mechanisms both morphological and chemical to protect themselves against herbivores.
Morphological defence mechanisms :
• Cactus plants (Opuntia) are modified into sharp spines (thorns) to deter herbivores from feeding on them.
• Sharp thorns along with leaves are present in Acacia to deter herbivores.
• In some plants, the margins of their leaves are spiny or have sharp edges that prevent herbivores from feeding on them. (Any one)
Chemical defence mechanisms :
• All parts of Caloptropis weeds contain toxic glycosides, which can prove to be fatal if ingested by herbivores.
• Chemical substances such as nicotine, caffeine,quinine and opium are produced in plants as a part of self-defence.

Question. Comment on the following diagrams: A, B, C, D, G, P, Q, R, S are species.

Answer: Fig. I: It is a single population and all individuals are of the same species, i.e., A individuals interact among themselves and their environment.
Fig. II: It is a community and it contains three populations of species A, B and C. They interact with each other and their environment.
Fig. III: It is a biome. It contains three communities of which one is in climax and other two are in different stage of development. All three communities are in the same environment and they interact with each other and their environment.

 Question. Name important defence mechanisms in plants against herbivory.
Answer: 

Question. Draw and explain a logistic curve for a population of density (N) at time (t) whose intrinsic rate of natural increase is (r) and carrying capacity is (K).
Answer: 

Question. (a) Explain with the help of a graph the population growth curve when resources are
(i) limiting and (ii) not limiting.
(b) “Nature has a carrying capacity for a species.” Explain. 
Answer: (a) QU There are two models of population growth:
(i) The exponential growth
(ii) Logistic growth
(i) Exponential Growth
QU The exponential or geometric growth is common where the resources (food + space) are unlimited.
QU Each species has the ability to realise fully its innate potential to grow in number.
QU The equation for exponential growth can be derived as follows: to
(b) (i) The resources become limited at certain point of time, so no population can grow exponentially.
(ii) Every ecosystem or environment or habitat has limited resources to support a particular maximum number of individuals called its carrying capacity (K).

Question. Write a short note on:
(a) Adaptations of desert plants and animals
(b) Adaptations of plants to water scarcity
(c) Behavioural adaptations in animals
(d) Importance of light to plants
(e) Effect of temperature or water scarcity and the adaptations of animals.
Answer: (a) Adaptations of desert plants are as follows:
(i) Desert plants have cuticles to minimise transpiration.
(ii) In some desert plants, leaves are modified into spines to minimise loss of water.
(iii) They have long roots and adaptations to reduce transpiration, e.g., Acacia.
(iv) Stomata are present in deeppits.
Adaptations of desert animals are as follows:
(i) Desert animals have concentrated their urine for minimum loss of water, e.g., Kangaroo rat.
(ii) Desert animals absorb heat from the sun, when the body temperature drops below the comfort zone.
(iii) They live in burrows during hot season and have little water requirement, e.g., camel.
(iv) Meet water requirement by interval fat oxidation.
(b) Adaptations of plants to water scarcity
(i) Some desert plants develop special photosynthetic pathway (CAM) to minimise the loss of water and close stomata during day.
(ii) Some desert plants have sunken stomata to minimise the loss of water.
(iii) Epidermis is thick walled with thick cuticles and often possess wax, thus, reducing thesurf ace transpiration.
(iv) Roots are deep-seated, almost reaching the water table, e.g., Prosopis.
(v) These xerophytes possess hard and pointed spines (modified leaves) to reduce transpiration.
(c) Behavioural adaptations in animals
(i) Desert lizards bask in the sun and absorb heat when their body temperature drops below the comfort zone, but move into shade when the ambient temperature starts increasing.
(ii) Some species are capable of burrowing into the soil to hide and escape from ground heat.
(iii) Hibernation and aestivation are quite common in ectothermal animals.
(d) Importance of light to plants
(i) Light is important for manufacturing food by the process of photosynthesis.
(ii) Duration of light determines flowering and fruit formation.
(iii) Light also determines the temperature which is associated with functioning of enzymes.
(iv) Light is essential for growth and development of plant because it provides organic materials.
(e) Effect of temperature or water scarcity and the adaptations of animals.
(i) Animals living in arid areas reduce water loss to minimum. For example, Kangaroo Rat feeds on dry seeds and seldom drink water.
(ii) The requirement of water is often compensated by food and metabolic water. Water loss is prevented by burrowing into the soil to hide and escape from the above ground heat, concentration of urine and solid faeces. Camel stops producing urine when water is not available and can remain without water for many days.
(iii) Animals protect themselves from excessive cold by deposition of fat, fur, etc. Bears undergo hibernation during winters.

Question. (a) “Organisms may be conformers or regulators.” Explain this statement and give one example of each.
(b) Why are there more conformers than regulators in the animal world?
Answer: (a) Conformers are organisms which cannot maintain a constant internal environment under varying external environmental conditions. They change body temperature and osmotic concentration with change in external environment. For example, all plants, fishes, amphibians etc.
Regulators are organisms which can maintain homoeostasis (by physiological means or behavioural means) i.e., they maintain constant body temperature and osmotic concentration.
For example, birds and mammals.
(b) Thermoregulation is energetically expensive for animals. Therefore, more conformers are found.

Question. Distinguish between the following:
(a) Hibernation and aestivation
(b) Ectotherms and endotherms
Answer: (a) Differences between hibernation and aestivation

Question. (a) Following are the responses of different animals to various ecological factors. Describe each one with the help of an example
(i) Regulate
(ii) Conform
(iii) Migrate
(iv) Suspend
(b) If 18 individuals in a population of 80 butterflies die in a week, calculate the death rate of population of butterflies during the period.
Answer: (a) (i) Regulate : Maintain constant internal temperature / osmotic concentration / homeostasis.
e.g., birds / mammals.
(ii) Conform : Do not maintain constant internal temperature / osmotic concentration / No homeostasis.
e.g., any one example of animal other than birds and mammals.
(iii) Migrate : Temporary movement of organisms from the stressful habitats to hospitable areas and return when stressful period is over.
example : Bar headed geese
(iv) Suspend : Reducing / minimising the metabolic activities during unfavourable conditions.
e.g. Polar bear / amphibian / snails / fish / any other example of animals .
(b) Death rate = 18/80 = 0.225 therefore, death rate percentage will be = 0.225 × 100 = 22.5% butterfly death per week
• Predators help in maintaining species diversity in a community, by reducing the intensity of competition among competing prey species.